The invention is intended mainly for use in integrated circuit chips where both digital and analog functions are employed simultaneously. Analog, or linear, circuits are employed in conjunction with RTL, TTL, ECL or CMOS logic configurations in many applications. The logic circuit choice is based upon the desired performance characteristics and the analog circuits are selected to provide the required function and to be compatible with the logic circuit manufacturing process.
Typically, an integrated circuit is designed to operate at a specified supply voltage, but it will function normally over a range of supply voltages. Unfortunately, when a logic system is operated at a low voltage it can produce false outputs and thus perform incorrectly. Accordingly, it has become standard practice to provide a circuit function that responds to a low supply voltage condition and shuts off or locks out the digital circuit outputs when the low voltage condition exists. Actually, while the invention mainly relates to the lockout of digital circuits, it can also be applied to linear circuits alone.
One of the best ways of providing a low voltage lockout is to specify or identify a low voltage state and then provide a circuit that will reliably sense it and produce a signal that can be used for the electrical discontinuance of circuit operation. Specifying such a voltage can be a problem because the circuit response can result in a low voltage response tolerance. Also, the circuit that responds to the low voltage can have a tolerance. These combined tolerances can produce a large range of uncertainty so that the circuit design must take into account all of the tolerances involved and respond in such a way that successful lockout will occur under all conditions. These tolerances are exacerbated by temperature effects that must be taken into account. The result is that the low voltage response must be conservatively applied and is therefore considerably higher than would be required for most conditions.
One well known application of combined lineardigital circuitry is the motor control chip. In this device a motor is controlled by the use of high efficiency switching-mode or digital controllers operated by pulses created in response to linear circuitry. It is important to prevent the production of false pulses if the motor is to remain off when it is supposed to be off. Additionally, it is important that the motor not be commanded to drive simultaneously in both forward and reverse directions which could damage the motor and/or its controllers.